The present invention relates to multicolor organic light-emitting materials and devices constructed using such materials. More specifically, the present invention relates to the use of oligo(phenylenevinylene)s in organic light-emitting devices (OLED).
Electronic displays are utilized in television sets, computer terminals and in a host of other applications. Such a medium offers speed, versatility, and interactivity. Manufacturers of electronic devices are working to develop displays that provide brighter, sharper pictures and provide a high resolution full color display at good light level and at competitive pricing.
The drive to improved displays has lead to the development of liquid crystal displays (LCDs). LCDs operate fairly reliably. However, LCD technology has a number of shortcomings, including weak brightness, relatively low contrast and resolution, large power requirements and high power backlighting requirements.
One alternative to LCDs are light-emitting displays (LEDs). Light-emitting displays make use of thin film materials which emit light when excited by electric current. Light-emitting displays are often fabricated using inorganic materials, such as manganese (Mn)-doped zinc sulfide (ZnS). Although inorganic light-emitting displays can provide high performance and durability, they suffer from large power requirements and expensive manufacturing.
Organic light-emitting displays (OLEDS), on the other hand, can be realized in a flexible form. Their electroluminescent wavelength can be selected from a wider range than inorganic displays. Furthermore, organic electroluminescent devices can easily be fabricated by means of a coating technique and large devices of this type can be easily produced at low cost. Another advantage is that a low voltage can drive organic electroluminescent devices.
OLEDs, whose structure is based upon the use of layers of organic optoelectronic materials, generally use radiative recombination of excitons as a mechanism to produce optical emission. OLEDs are typically comprised of at least two thin organic layers between an anode and a cathode, as shown in FIG. 1. The material of one of these layers is specifically chosen based on the material""s ability to transport holes, a xe2x80x9chole transporting layerxe2x80x9d (HTL) 13, and the material of the other layer is specifically selected according to its ability to transport electrons, an xe2x80x9celectron transporting layerxe2x80x9d (ETL) 17. The electroluminescent material may be present in a separate emissive layer (EML) 15 between the HTL and the ETL in what is referred to as a xe2x80x9cdouble heterostructurexe2x80x9d (DH). With the application of an electric potential (typically 100 MV/m), the anode 11 injects holes (positive charge carriers) into the HTL 13, while the cathode 19 injects electrons into the ETL 17. The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize in the EML 15 on the same molecule, a Frenkel exciton is formed. These excitons are trapped in the material which has the lowest energy. Recombination of the short-lived excitons may be visualized as an electron dropping from its conduction potential to a valence band, with relaxation occurring, under certain conditions, preferentially via a photoemissive mechanism.
Alternatively, the materials that function as the ETL or HTL of an OLED may also serve as the medium in which exciton formation and electroluminescent emission occur. Such OLEDs are referred to as having a xe2x80x9csingle heterostructurexe2x80x9d (SH).
Various compounds have been used as HTL materials or ETL materials. HTL materials often consist of triaryl amines in various forms which show high hole mobilities (approximately 10xe2x88x923 cm2/Vs). The most common hole transporter used in the HTL of OLEDs is a biphenyl bridged diamine, N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-methylphenyl)-1,1-biphenyl-4,4xe2x80x2-diamine (TPD). Aluminum tris-(8-hydroxyquinolate) (Alq3) is the most common ETL material, and others include oxidiazole, triazole, and triazine.
Various compounds have been employed as electroluminescent emitters. Such compounds may be present in the EML, separate from the ETL and HTL layers. Alternatively, such compounds may be combined into or be the same as the compounds used in the ETL or HTL layers.
Burroughes et al., Nature, Vol. 347, 1990, pp. 539 describe the use of a highly fluorescent conjugated polymer, poly (phenylenevinylene) (PPV), as the active material in a single-layer OLED, in which a thin layer of the active organic material is sandwiched between two electrode. The use of PPV in electroluminescent devices is also described in U.S. Pat. No. 5,247,190 (issued Sep. 21, 1993). Both references are incorporated herein by reference. Some difficulties associated with PPV include the use of wet processes such as spin-coating, difficulties in fabricating multilayer devices for confined structures, and emission peaks in the green, or green-yellow regions of the spectrum, rather than in the blue.
Initially synthesized as model compounds in order to gain more insight into the structural and electronic peculiarities of the corresponding polymers, conjugated oligomers have also been investigated as potential materials in electrooptical applications. The use of oligo(phenylenevinylene)s (OPV), oligo(phenylene)s, and oligothiophenes, for example, has been investigated (see a review article on OLEDs by Mitschke et al. J. Mater. Chem. 2000, Vol. 10, 1471-1507, incorporated by reference herein).
The shorter conjugated segment of OPVs compared with PPVs improves the fluorescence yields and the electroluminescent efficiency, and the wavelength for absorption and emission decreases with decreasing conjugated length.
However, there still exists a need for efficient and stable materials, especially blue emitters, such as in the 420-450 nm range.
The use of oligo(phenylenevinylene) (OPV) and its derivatives in light-emitting devices (LEDs) is disclosed. Also disclosed are light-emitting devices made from oligo(phenylenevinylene)s, as well as displays employing such devices.
Thus, according to one aspect, the invention provides a light-emitting device comprising an anode; a cathode; and an emissive layer between the anode and cathode, comprising an oligo(phenylenevinylene) of the Formula (I): 
wherein
n is 0 to 8;
X is H, NHR, NR2, (O(CH2)2)2OR, OR, NO2, or SO2R;
Y is H, NHR, NR2, (O(CH2)2)2OR, OR, NO2, or SO2R;
a, b, c, and d are each independently selected from the group consisting of H, R, OR, NHR, and NR2;
e and f are each independently selected from the group consisting of H, SiR3, alkyl, and aryl groups; and
each R is independently selected from the group consisting of a substituted and unsubstituted alkyl and aryl groups; and
wherein if both X and Y are OR, at least one of a, b, c, d, e, and f is other than H; if both X and Y are one of (O(CH2)2)2OCH3 and SO2C6H13, at least one of a, b, c, d, e, and f is other than H; if X, Y, e, and f are all H, at least one of a, b, c, and d is other than H, and a, b, c, and d are not all tert-butyl; and if X, Y, a, b, c, and d are all H, e and f are not both C6H13 or both C8H17.
Also disclosed is a display employing the device.
In another aspect, the invention provides a method of making a light-emitting device, the method comprising the steps of preparing an oligo(phenylenevinylene) of the Formula (I); and layering the oligo(phenylenevinylene) of the Formula (I) between an anode and a cathode to form a light-emitting diode.
In a further aspect, a method of the invention further comprises the step of selecting n, X, Y, a, b, c, d, e, f, and R of the OPV of Formula (I) to selectively tune the emissive wavelength of the OPV to a desired wavelength
In another aspect, the invention provides the use of an oligo(phenylenevinylene) according to Formula (I) for electroluminescent emission in a light-emitting device.
There are many advantages in using low molecular weight OPVs of the invention for light-emitting devices. First, OPVs can be sublimated in vacuum, which eliminates the use of organic solvents and makes multilayer structures possible. Secondly, the emission peak of OPVs can be shifted to the blue or green-blue regions of the spectrum by reducing the length of the xcfx80-conjugated backbone. Thirdly, OPVs have well-defined chain lengths and molecular structure, that makes data analysis easier than on polymer system, and provides a better basis for understanding the relationship between the molecular structures and the chemical/physical properties.
Other aspects and advantages of embodiments of the invention will be readily apparent to those ordinarily skilled in the art upon a review of the following description.